One of the hardest hit areas, Moore, Okla., is no stranger to devastating twisters. A massive tornado that hit on May 3, 1999, was one of the most powerful and destructive single tornadoes in history until the 2011 tornado that hit Joplin, Mo., killing 158 people and causing an estimated $2.8 billion in damage.

Monday's tornado, which is estimated to have been on the ground for nearly 40 minutes and possibly more than a mile wide, followed closely the track of the one that struck in 1999, according to the National Weather Service.

Tornado strength is currently measured on what is called the Enhanced Fujita Scale (adapted from the simpler Fujita Scale in 2007), which gives the tornado a rating from 0 to 5 based on estimated wind speeds and the severity of the damage. The 1999 Moore tornado is listed as an F-5, the most powerful, though it is still unclear where on the scale Monday's tornado will be until the damage can be examined.

Below is a quick rundown of the EF scale and damage estimates, according to Weather Underground: